Nacelle structures for jet engines of an aircraft provide a housing within which the jet engine is supported. The nacelle structure typically includes two substantially semi-circular halves that are coupled together to shroud the jet engine core. The nacelle structure also typically includes a thrust reverser that can provide assistance in slowing the aircraft by redirecting the engine thrust. The thrust reverser includes a panel of the nacelle that is translated between a stowed position, for normal operation during flight, and a deployed position, for redirecting the engine thrust, such as during landing of the aircraft. The thrust reverser panel slides along a track beam to move between the stowed position and the deployed position.
Thrust reverser track beams support the thrust reverser panel during translation and join the thrust reverser panels to internal fixed structure and to an engine pylon for under-wing carriage of the nacelle structure. Other installations of jet engines may enclose the engine core within a fuselage portion of the aircraft, in which case the track beams join the thrust reverser panels to the aircraft fuselage.
Each nacelle half is typically coupled to an engine pylon at the top of the nacelle structure by a hinge beam and is typically coupled to a latch structure at the bottom of the nacelle by a latch beam. The nacelle halves pivot about the hinge beams and are latched together by the latch beams.
Within the space enclosed by the nacelle structure, a bond panel is attached to the inner side of each nacelle half, placed between the engine core and the nacelle half. Each bond panel extends generally from the top of each nacelle half at the hinge beam to the bottom of each nacelle half at the latch beam. A bypass duct for airflow is formed between the outward-facing bond panel surface and the inner surface of each nacelle half. The bond panel of each respective nacelle half is coupled to the hinge beam along a longitudinal edge at the top of the panel and is coupled to the latch beam along a longitudinal edge at the bottom of the panel.
A conventional bond panel has a sandwich construction of an aluminum honeycomb core and a flat composite skin layer attached to the honeycomb core on the inner honeycomb side (toward the engine core) and a flat composite skin layer attached on the outer honeycomb side (toward the bypass duct). The honeycomb core has multiple honeycomb cells. The inner surface of the bond panel may be covered with a thermal blanket, to protect the bond panel and other components from the heat of the engine core.
The composite skin layer of the bond panel's outer honeycomb side, which generally forms part of the bypass duct, typically includes perforations in the skin. The perforations are arranged so that at least one perforation is aligned with each cell of the underlying honeycomb core. The perforations and honeycomb core make use of the well-known Helmholtz resonator effect for noise attenuation.
The size of the honeycomb cells may vary between the curved surface areas of the bond panel and the flatter regions of the bond panel, which are located between the curved surface and the attaching beams, and are referred to as bifurcation regions. The honeycomb cell volumes of the individual cells in the bifurcation regions are significantly reduced as compared to the cell volumes outside of the bifurcation regions, often by a factor of twenty times or more, to account for the severe loads on the bond panel in the bifurcation regions.
The strength requirements for the bond panel in the bifurcation regions call for an increased density honeycomb core and a lack of composite skin perforations in the bifurcation regions. The density of the honeycomb core is increased by utilizing cells having smaller volume (i.e., tighter spacing), or by increased wall thickness, or both. The perforations are omitted in the bifurcation regions because the perforations would otherwise compromise the structural integrity of the bifurcation regions beyond what would be acceptable for a conventional construction, given the structural loads to the bifurcation regions. As noted above, the bond panel is coupled to the hinge beam and latch beam along outer edges of corresponding bifurcation regions. Each panel-to-beam coupling is achieved by riveting the bond panel to a planar surface of an extending wall of the respective beams. To ensure a secure coupling, typically two rows of rivets extending along the beam length are used. For many bond panels, in excess of one hundred rivets are required to fasten the bond panel to a beam. The rivets are not compatible with perforations in the bifurcation regions, which would unduly weaken the bond panel.
The increased honeycomb density and the lack of composite perforations in the bifurcation regions result in less than optimum noise attenuation characteristics and, coupled with the rivets, undesired weight for the bond panel and nacelle structure. Unfortunately, a not insignificant area of the bond panel in the bifurcation regions are part of the bypass duct. The reduced noise attenuation can result in difficulty meeting noise regulations for ground operation and can produce a louder, less enjoyable flight experience for passengers.
There is a need for improved bond panel construction that would increase the noise attenuation and reduce costs and weight of the bond panel. The present invention addresses that need.